Electric power steering apparatus, which gives assist power by means of auxiliary force of motors in order to enable smooth operation of steering wheel in automobile, is often used. The electric power steering apparatus gives drive power of motor as assist power to steering shaft or rack shaft using transmission mechanisms such as gear or belt via reduction gears. FIG. 1 illustrates a simple construction of such an electric power steering apparatus. A shaft 202 of a steering wheel 201 is jointed to a tie rod 206 of a front wheel via a reduction gear 203, universal joints 204a and 204b and a pinion rack mechanism 205. The shaft 202 is provided with a torque sensor 207 that detects steering torque of the steering wheel 201, and a motor 208 that assists the steering power of the steering wheel 201 is connected to the shaft 202 via the reduction gear 203.
FIG. 2 illustrates one example of control of motor drive to be used in such an electric power steering apparatus.
In the control of the motor drive, generally, an analog signal detected by a sensor is converted into a digital signal, and digitally controlled by a digital controller such as a microcomputer. In FIG. 2, a torque T detected by a torque sensor 207 and a vehicle speed V detected by a speed sensor, not shown, are converted into digital values, and the digital values are inputted into a current command value calculating unit 10 as a current command value calculating means so that a current command value Iref is calculated. In this case, the current command value Iref calculated by the current command value calculating unit 10 is zero-order-held by a zero-order hold circuit 14 as a zero-order hold means, so that a current command value Iref0 is output. The zero-order hold circuit 14 and its problem are explained in detail later.
A subtracting unit 20 calculates a difference ΔI between an actual motor current Im detected by a current detecting circuit 28 and the current command value Iref0. A current control means such as a current control unit 22 is used to control so that the difference ΔI is eliminated. A voltage command value Vref as an output from the current control unit 22 is inputted into a PWM control unit 24 as a PWM control means, and the PWM control unit 24 outputs a PWM signal to an inverter circuit 26 as one example of a motor driving circuit which supplies a current to the motor 208.
As shown in FIG. 2, a portion which is surrounded by a broken line A including the current command value calculating unit 10 through the PWM control unit 24 is comprised of a digital control means such as a microcomputer. That is to say, at least, the digital control means is comprised of a plurality of control means such as the current command value calculating means, the current control means and the PWM control means.
The motor 208 is PWM-driven based on the PWM signal, and a motor current Im is supplied from an inverter circuit 26 to the motor 208 so that the inverter circuit 26 conforms to the current command value Iref0.
As mentioned above, since the motor drive of the electric power steering apparatus is digitally controlled by the digital control means such as microcomputer, the motor is controlled by the current command value Iref0 which is zero-order-held. Examples of sampling periods of the current command value calculating unit 10 and the current control unit 22 are, for example, 2 ms and 1 ms, respectively, and thus they are different from each other. Further, the current command value Iref0, which is obtained by zero-order-holding the current command value Iref calculated by the current command value calculating unit 10 using the zero-order hold circuit 14, becomes a current command value which changes in a staircase pattern.
FIG. 3 illustrates one example of the current command value Iref0 which is zero-order-held. As shown in FIG. 3, even if the motor is controlled based on the current command value Iref0 having a staircase waveform, the motor cannot be energized by a motor current causing an abrupt change because conventionally the inductance value of the winding wire of the motor is comparatively large. As a result, the corner of the staircase-pattern current waveform is rounded off and smooth motor current is obtained.
Due to the reason explained below, in recent years, the inductance value of the winding wires of the motors used for the electric power steering apparatus becomes smaller, and accordingly the problem, mentioned later, arises.
In recent years, the high power and high efficiency electric power steering apparatus is required. However, a power source voltage of a vehicle is 12V, it is difficult to heighten a voltage for high output and high efficiency. In motors itself which are applied to electric power steering apparatus, the inductance value of winding wires tend to be small in order to heighten output and efficiency.
As a result, as shown in FIG. 4, since the inductance value of a winding wire of a motor is small with respect to the current command value Iref0 of staircase pattern which is zero-order-held, a waveform of the motor current Im to energize the motor also becomes a staircase-patterned waveform according to the waveform of the current command value Iref0. The actual motor current Im having the staircase-patterned current waveform includes many higher harmonic waves, and the higher harmonic wave causes noises. In other words, a quantization error in the digital control causes higher harmonic wave included in the actual motor current Im, thereby causing the problem of noise.
The inductance value of the winding wire of the motor becomes small, and further, in the electric power steering apparatus, a small-sized brushless DC motor with high output and rectangular wave current tends to be used increasingly in recent years. The rectangular wave current includes a portion where di/dt is large, and a quantization error obviously appears in the current portion with large di/dt. In the case, therefore, where the motor is driven by the rectangular wave current, the quantization error in the current portion of the rectangular wave current with large di/dt also causes an increase in higher harmonic waves, thereby causing the increase in noise.
In the electric power steering apparatus, since a battery voltage is 12V, namely low, when the output is tried to be increased, the current becomes high, and it is necessary to flow the current of maximally about 100 A. In order to supply high current, therefore, di/dt of the motor current becomes large as a result, and thus the above-mentioned problem is increased.
Here, there is a Patent Document (Japanese Patent Application Laid-Open No. 2000-018069), which treats the problem similar to that the sampling periods of the respective control means in the digital control means are different. However, Japanese Patent Application Laid-Open No. 2000-018069, does not refer to an electric power steering apparatus but refers to a throttle valve control unit for automobile. FIG. 5 illustrates a control block diagram of the throttle valve control unit for automobile disclosed in Japanese Patent Application Laid-Open No. 2000-018069. As shown in FIG. 5, feedback control is performed by a throttle opening signal as a feedback signal from an opening sensor 303 so that a check is made whether a motor 308 for opening a valve operates according to an opening command of the throttle valve.
Construction of the control block is as follows. The opening command is inputted into an opening control unit 301 comprised of a low-speed microcomputer via an interface 302, and a throttle opening signal detected by the opening sensor 303 is inputted into the opening control unit 301 via an operational amplifier 304. A current command calculated by the opening control unit 301 based on these input values and forward/reverse rotation signal are output. The forward/reverse rotation signal is inputted directly to a chopper 307 composed of an H bridge. Meanwhile, the current command is inputted into a current control unit 305 composed of an analog circuit via a filter 306. The current control unit 305 outputs a PWM signal which is calculated based on the current command via the filter 306 and a current (current detection signal) detected by a resistance 309 for detecting current, and the chopper 307 is controlled based on the PWM signal so that the current is supplied from the chopper 307 to the motor 308.
The opening control unit 301 is composed of a digital circuit of the low-speed microcomputer, and the current control unit 305 is composed of a high-speed analog circuit. This is because although the current control unit should perform an operation at high speed, the microcomputer which can calculates at high speed is expensive. For this reason, the opening control unit 301 is composed of the inexpensive low-speed microcomputer, whereas the current control unit 305 which requires high-speed operation is composed of the inexpensive high-speed analog circuit so that the high-speed control can be made inexpensively in the entire device. Since it is not, however, preferable that the digital signal is inputted directly into the analog circuit, it is inputted via the filter 306 as a low-pass filter composed of a capacitor and a resistance.
In the throttle valve control device for automobile disclosed in Japanese Patent Application Laid-Open No. 2000-018069, even if a simple low-pass filter is used, a phase delay is large, and thus a high-speed response cannot be realized in an entire control system. Further, this publication does not refer to the problem relating to the motor where the inductance value of the winding wire is small, and namely, does not disclose any solving means in the case where a motor driving device using the motor with winding wire of small inductance value and an electric power steering apparatus having such a motor driving device are controlled digitally.
As mentioned above, in the motor driving device where the motor having the winding wire of small inductance value is controlled by a digital control means comprised of a plurality of control means such as a current command value calculating means, a current control means and a PWM control means whose sampling periods are different, when the current command value Iref calculated by the current command value calculating means is zero-order-held, the zero-order-held current command value Iref0 has a staircase pattern waveform, and thus also the waveform of a current to energize the winding wire of the motor is similar to the staircase waveform of the current command value Iref0. The staircase-pattern current waveform of the motor current Im includes a lot of higher harmonic wave components, and the higher harmonic wave current causes noise. Further, also in the electric power steering apparatus having the motor driving device, the noise makes a driver and/or passengers uncomfortable.
The present invention is made to solve the above problem, and its object is to provide a motor driving device in which a motor having a winding wire of small inductance value is controlled by a digital control means comprised of a plurality of control means such as a current command value calculating means, a current control means and a PWM control means whose sampling periods are different, and which reduces a quantization error so as to suppress higher harmonic wave (higher harmonic wave components) of a motor current and reduces motor noise, and to provide an electric power steering apparatus having the motor driving device.